1. Field of the Invention
The present invention relates to a light-emitting element, a light-emitting device, a lighting device, and an electronic device, each of which utilizes electroluminescence of an organic material.
2. Description of the Related Art
An organic compound can take a wider variety of structures than an inorganic compound, and have a possibility to synthesize a material having various functions by appropriate molecular-design of an organic compound. Owing to those advantages, electronics utilizing a functional organic material has been attracting attention in recent years.
For example, a solar cell, a light-emitting element, an organic transistor, and the like are exemplified as electronic devices utilizing an organic compound as a functional material. These are devices taking advantage of electric properties and optical properties of the organic compound. Among them, in particular, a light-emitting element has been making remarkable development.
The light emission mechanism of a light-emitting element is as follows: electrons injected from a cathode and holes injected from an anode recombine in the luminescent center of a light-emitting layer to form molecular excitons by applying a voltage between a pair of electrodes with the light-emitting layer interposed therebetween; and when the molecular excitons relax to a ground state, energy is released to emit light.
Here, it is said that the internal quantum efficiency of a light-emitting element depends on the recombination efficiency, the exciton generation efficiency, the quantum efficiency, and the like of carriers.
Among them, in the exciton generation efficiency, quantum physical chemistry shows that the ratio of a triplet exciton and a singlet exciton, which are generated, is statistically 3:1. For this reason, the maximum internal quantum efficiencies of a light-emitting element using a fluorescent light-emitting material and a light-emitting element using phosphorescent light-emitting material can be 0.25 and 1, respectively.
The recombination efficiency shows the probability of recombination of carriers which are injected from one electrode and do not pass through the other electrode. In order to improve the recombination efficiency, a block layer for trapping carriers is provided with respect to holes or electrons in some cases.
The internal quantum efficiency is defined by the rate of generated photons with respect to the number of electrons injected to a light-emitting element. An improvement of the internal quantum efficiency improves luminous efficiency and reduces power consumption; therefore, many researches have been made in aspects of a material and an element structure (for example, see Patent Document 1).